Scintillation counting of a beta emitting radioactive isotopes is a standard method of measuring the level of a labeled substance. Scintillation counters are commercially available for such analysis. As disclosed in U.S. Pat. No. 5,512,753, a number of techniques are available for providing a scintillant. These include solid scintillators comprising crystals of a solid hydrocarbon material; liquid scintillators which have one or more solid scintillators dissolved in a liquid solvent; solid solution scintillators wherein a solid scintillator is embedded in a solid polymer and solid scintillators which comprise crystals of a suitable inorganic material. To this list, the '753 patent adds encapsulated scintillators which include a liquid scintillator core within a shell.
Applications of scintillation counting incorporated into specifically designed assays have been termed "scintillation proximity assays." These depend on the necessity for the scintillant to be within a certain distance of a beta emitting radioactive isotope in order to emit light; the mean radiation distance of the beta emitter determines the distance required for the scintillant to emit detectable light. By associating the scintillant with a reagent to be tested against a labeled substance, the interaction of the substance with reagent can be assessed. Either the analyte bears a beta emitting radioisotope, or is placed in competition with a labeled form or analog of the analyte for the reagent associated with scintillant. Such assays are described generally in U.S. Pat. Nos. 4,382,074; 4,568,649; and 4,687,636. A typical approach involves a homogeneous immunoassay system wherein a scintillant is embedded in a particulate bead which is conjugated to an antibody. Substances immunoreactive with the antibody can be measured by competition with a radioactively labeled form of the substance or its analog for the antibody coupled to the beads; the level of light emitted by the scintillant is thus inversely proportional to the analyte in the sample.
A refinement of this concept is disclosed in U.S. Pat. No. 5,246,869, wherein several ligands can be assayed simultaneously. This assay employs two or more different scintillants which can be distinguished by their emission spectra, each attached to different reagents which specially bind different ligands.
U.S. Pat. No. 5,665,562, incorporated herein by reference, describes a scintillation based system for monitoring uptake of radioactively labeled substrates by cellular monolayers. In this macroscopic method, a modified form of a 96-well microtiter plate is employed, whereby a region of the base portion of the wells is constructed of polystyrene containing a scintillant, such as 2-(4-t-butylphenyl)-5-(4-biphenylyl)-1-3,4-oxadiazole (2%) and 9,10-diphenylanthracene (0.5%). The plate is constructed so as to minimize or eliminate the passage of light from well to well. A monolayer of cells is coated in each well and the uptake of a radioactively labeled substrate is measured by the emission of light by the scintillant.
In addition, digital systems for readout and storage of emission of radiation have been described in U.S. Pat. No. 5,627,381.
However, to applicant's knowledge, the use of scintillants embedded in cells, tissues or organelles to follow the behavior of compounds labeled with beta emitting isotopes at microscopic resolution has not been previously employed. The availability of wide-field microscopy for observing emitted light with high resolution enhances this possibility, though ordinary microscopic techniques may also be used.